Assuming 5 gallons of 1.050 wort, you can assume about 125 gallons of CO2 (or 3.78*125 = 472.5 liters) to be produced.

Assuming 2 gallons (or 3.78 * 2 = 7.56 liters) of air in the headspace and uniform mixing of the fermentation CO2 with the headspace air, by the end of fermentation we have about 7.56/(472.5 + 7.56)*100% = 1.575% of this air remaining, or about 0.12 liters.

.12 liters of air weighs about .12 * 1200 = 144 mg, 21% of which is oxygen, leaving us with .21 * 144 = 30.24 mg oxygen trapped in the keg.

5 gallons of beer is 18.92 liters, so at minimum we have 30.24 / 18.92 = 1.6 mg/l TPO coming from the fermenter headspace. This alone is 16 times higher than the recommended maximum TPO in a commercial setting (0.1 mg/l), with most breweries shooting for far below that number. While developing the LoDO process, we measured that even 0.2-0.3 mg/l oxygen picked up post-fermentation resulted in significant and rapid flavor deterioration (on the order of a couple weeks at 3 degrees C)

Is it possible that some of this headspace oxygen diffuses into the wort and is consumed by the yeast during fermentation? Yes. How much will do that, I don't know. I'm not really inclined to start using one of these things.

If you always do what you've always done, then you'll always get what you've always gotten.